The invention relates to a corrector for correcting first-order chromatic aberrations of the first degree as well as geometric third-order aberrations of electron-optical lens systems.
A corresponding corrector is disclosed in German patent publication 4,204,512. The corrector includes a total of six quadrupoles and octopoles superposed on the quadrupoles. The potentials of the quadrupoles are excited anti-symmetrically to a center plane. An additional octopole is mounted in the center plane. The corrector defines a telescopic system. With this corrector, the third-order spherical aberration and the off-axis first-order chromatic aberration of the first degree of an electron-optical lens system can be corrected. A correction of the coma of a skewed beam is possible with this system in that the coma-free point of the lens, which is to be corrected, is placed at the nodal point of the corrector. The following cannot be corrected with this corrector: off-axis geometric third-order aberrations such as the astigmatism of skewed beams; the distortion; and, the image field curvature. In total, this corrective device can therefore be used only for systems having relatively small image fields such as are conventional in transmission electron microscopes. In contrast, this corrective device cannot be used in combination with systems which require large image fields such as in electron projection lithography.
It is an object of the invention to provide a corrector with which the axial and off-axis energy-dependent first-order aberrations of the first degree of an electron-optical lens system as well as the third-order spherical aberrations can be corrected and which can also be expanded to correct: the distortion, image field curvature, and the astigmatism of skewed beams of the electron-optical lens system. In an advantageous embodiment of the invention, the axial and off-axial energy-dependent first-order aberrations of the second degree and all geometric third-order aberrations should be correctable at least for a specific plane.
The corrector of the invention is for correcting energy-dependent first-order aberrations of the first degree and third-order spherical aberrations of an electron-optical lens system. The corrector includes: a quadrupole-septuplet including at least seven quadrupoles defining a center plane (ZS); the quadrupoles being excitable symmetrically to the center plane (ZS) along a linear axis; and, at least five octopoles excitable within the quadrupole-septuplet.
The corrector according to the invention includes at least seven quadrupoles, that is, a quadrupole-septuplet along a linear axis which are excitable symmetrically to a center plane along the linear axis. Furthermore, at least five octopole fields can be excited in the corrector of the invention at various locations within the corrector.
According to the invention, it was recognized that, in a system having only seven quadrupoles, all axial or off-axial fundamental paths can run either symmetrically or antisymmetrically to the center plane of the corrector. With this symmetry, it is possible to correct spherical aberrations without additionally introducing coma and distortion. If the lens system, which is to be corrected, has a coma, the zero crossovers of the field paths of the corrector are to be placed at the coma-free point of the lens system or lens so that the entire system is also free of coma.
Of the total of seven quadrupoles, at least three should be electromagnetic, that is, there should be a superposition of electrostatic and magnetic quadrupole fields. With the electromagnetic quadrupoles, the correction of the axial energy-dependent aberrations is possible without introducing off-axial energy-dependent aberrations. Because of the symmetry of the quadrupoles, no off-axial energy-dependent aberrations are introduced in the nodal plane in which the field rays have a zero crossover. This nodal plane thereby defines an achromatic plane of the corrector and simultaneously a coma-free plane of the corrector.
If an arrangement of the nodal plane of the corrector is not possible in the achromatic plane of the imaging system, which is to be corrected, then off-axial energy-dependent aberrations can be obtained via a suitable distribution of the magnetic and electric quadrupole intensities in that the electric quadrupole intensities have a component, which is antisymmetric to the symmetry plane of the septuplet which is in magnitude equal but of inverse polarity to a component of the magnetic quadrupole intensities which is antisymmetric to the symmetry plane of the septuplet. The total quadrupole intensity is then again symmetrical to the symmetry plane of the septuplet. The symmetrical component of the electric and magnetic quadrupole intensities is so adjusted with respect to the ratio of the intensities to each other that the axial chromatic aberration is corrected and the antisymmetrical component of the electric and magnetic quadrupole intensities is so adjusted in the ratio of the intensities to each other that the off-axial chromatic aberrations are corrected.
The octopoles should be excited symmetrically to the center plane of the quadrupole septuplet so that the corrector does not introduce an additional coma and distortion.
In order to correct the three components Of the spherical aberration as independently as possible from each other, a first octopole pair should be excitable in the region of the first and last quadrupole, a second octopole pair should be excitable in the region of the second and sixth quadrupole and a third octopole should be excitable in the region of the center quadrupole. As an alternative to the excitation of the octopole in the region of the center quadrupole, it is however also possible to excite an additional octopole pair in the region of the third and fifth quadrupoles. An especially favorable decoupling of the components of the spherical aberration is achieved when the octopole fields are spatially superposed on the quadrupole fields; however, this is not absolutely necessary and especially slight position deviations are not critical.
It is especially advantageous to mount two geometrically equal quadrupole septuplets serially one behind the other along the linear axis. With such an arrangement, it is then possible to correct all geometric third-order aberrations as well as the axial energy-dependent first-order aberrations of the first degree and first-order aberrations of the second degree. An especially decoupled correction of all geometric third-order aberrations is achieved when an additional octopole is excitable in the mid plane between the quadrupole septuplets.
The corrector has two specific excitation modes for the octopoles: in a first specific excitation mode, the octopole fields are excitable in each quadrupole septuplet symmetrically to the center plane of the particular quadrupole septuplet and the octopoles of both quadrupole septuplets are excited symmetrically to the center plane between the quadrupole septuplets. In this mode of operation, the corrector is free of coma and free of distortion, that is, the corrector does not introduce additional coma or distortion.
In the second specific excitation mode, the octopoles in each quadrupole septuplet are excited antisymmetrically to the center plane of the particular quadrupole septuplet and the octopole fields of both quadrupole septuplets are excited antisymmetrically to the plane between the quadrupole septuplets. In this operating mode, no additional spherical aberration, no image field curvature and no astigmatism of skewed beams are additionally generated by the corrector. The operating mode selected is dependent upon the symmetry characteristics of the electron-optical lens system to be corrected. The image field curvature and the spherical aberration of electron-optical round lens systems are unavoidable. For this reason, in practice, the excitation of the octopole fields always has a symmetrical component. The geometric third-order aberrations of any round lens system can be, in principle, corrected by suitable superposition of symmetrical and antisymmetrical components of the octopole excitations.
Furthermore, it is advantageous to mount a further octopole which can be excited in the mid plane between the quadrupole septuplets. With this additional octopole together with the excitations of the two octopoles in the symmetry planes of the quadropole septuplets, it is possible to correct the spherical aberration substantially independently of off-axial image aberrations.
Furthermore, it is advantageous to generate the octopole fields or at least a portion of the octopole fields by the excitation of twelve poles or multipoles of higher order so that the orientation of the octopole fields is rotatable electrically about the linear axis. In this way, it is possible to correct the azimuth components of the coma, of the astigmatism of skewed beams and of the distortion.
Furthermore, it is additionally possible to also generate, in addition to the quadrupole fields and octopole fields, hexapole fields with the aid of the twelve poles by correspondingly exciting the individual multipole fields. With the aid of the hexapole fields, a portion of the fifth-order aberrations can be compensated.
Furthermore, it is advantageous when at least a portion of the octopole fields are electromagnetic, that is, likewise comprise a superposition of crossed electric and magnetic octopole fields. In this way, the energy-dependent first-order aberrations of the third degree can be partially corrected and therefore reduced.
Furthermore, it is advantageous to generate six multipole fields in order to correct the dominant geometric fifth-order aberrations.
The axial and off-axial fundamental paths should be symmetrical or antisymmetrical to the center plane of the particular quadrupole septuplet in each quadrupole septuplet in the corrector according to the invention.
The course of the fundamental paths in the XZ section of the first septuplet corresponds to the trace of the corresponding fundamental paths in the YZ section of the second septuplet and vice versa as noted in German patent publication 4,204,412 referred to initially herein.
A corresponding corrector with which all third-order aberrations and the energy-dependent first-order aberrations of the first degree can be corrected includes at least fourteen quadrupole fields and fifteen octopole fields which are excitable along the linear axis at different suitable locations.
A corresponding corrector preferably is applicable within an electron-optical imaging system as used, for example, in electron projection lithography and images a first plane demagnitized into a second plane. Such an electron-optical imaging system has at least two lenses and the corrector is mounted between these lenses.